White noise is random noise that has a flat spectral density — that is, the noise has the same amplitude, or intensity, throughout the audible frequency range (20 to 20,000 hertz). White noise is so named because it's analogous to white light, which is a mixture of all visible wavelengths of light.

Since it includes all audible frequencies, white noise is often used to mask other sounds. For example, some people use white noise machines for sleep aids to block out annoying noises in the environment.